The present invention relates to electrohydraulically controlled motor vehicle transmissions, in which a change of the gear position takes place by controlling certain control elements, such as clutches and brakes. For this purpose, the hydraulically actuated control elements establish the frictional connection between certain sets of gearwheels and a driving engine of the motor vehicle or the housing of the transmission.
A transmission of this type is described in Bosch Technische Berichte 7 (1983) 4, Pages 160 to 166 and in ATZ Automobiltechnische Zeitschrift 85 (1983) 6, Pages 401 to 405. By way of electromagnetic shift valves, the control elements are alternately connected with a pressure-controlled hydraulic circuit or with a pressure sink. As a function of operating parameters, the control apparatus also controls an electromagnetically actuated pressure control valve by means of which a pressure medium can be fed from a pressure source into the hydraulic circuit. A certain pressure, which is dependent on operating parameters, is adjusted in the hydraulic circuit by means of the pressure control valve.
According to principle, the shift valve, the pipes guiding the pressure medium downstream of the valve, and the control element proper have a variable filling volume which is not negligible. This filling volume has a fabrication variance and may be caused by aging, and vary as a function of operating parameters. Likewise, a variable actuating volume of the pressure medium, which is required for carrying out a necessary control movement, is to be fed to the control element. This actuating volume may have a fabrication variance and be caused by aging, and vary as a function of operating parameters.
The overall control of a control element is therefore composed of the phases "fill control element" and "actuate control element". The time required for changing the gear position is therefore the result of the addition of the time periods required for the two phases. However, low pressures in the hydraulic circuit, as they exist particularly in the case of downshifts in the coasting operation, may not provide a sufficiently fast filling of the participating hydraulic systems. This leads to the requirement of long time periods for a changing of the gear positions.
When the gear position is changed, a control element (of the gear position that is to be taken out ) that is to be shut off empties relatively rapidly so that the power transmission is interrupted almost immediately. Since a significant power flux can not yet exist in the control element to be actuated because of the extended filling time, the power flux between the driving engine and the transmission output as a whole can therefore be interrupted. Therefore, the rotational speed of the driving engine may fall to the rotational idling speed during the filling time of the control element.
Although the filling time could be shortened by a further increase of the pressure adjusted during the change of gear positions, this would result in significantly reduced shifting comfort because the clutches and brakes will then close very rapidly. This is exhibited by a more or less strong jolt in the transmission line and affects the longitudinal dynamics of the overall vehicle.
An object of the present invention is to provide a process for the control of the actuating pressure in a control element of an electrohydraulically controlled motor vehicle transmission in which the control elements to be controlled are uniformly filled substantially independently of the operating condition of the motor vehicle without any significant reduction of the power flux between the driving engine and the transmission output.
This and other objects are achieved by the present invention which provides a process for controlling actuating pressure in a control element of an electrohydraulically controlled transmission of a motor vehicle during a change of gear position. The automatic transmission has at least one electromagnetically actuated pressure control valve which is controlled by a control unit as a function of operating parameters and by means of which a pressure medium from a pressure source can be fed into a hydraulic circuit. A certain pressure can be controlled into the hydraulic circuit which is a function of operating parameters and which, during the change of the gear position, is increased by a certain amount. A shift valve is controlled by the control unit and connects the control element alternately with the pressure-controlled hydraulic circuit or with a pressure sink. The process comprises setting a pressure during the change of the gear position and controlling the control element with the pressure control valve in a time-limited manner out of a pressureless condition caused by the shift valve. This includes adjusting an increased pressure in the hydraulic circuit which has been increased with respect to the pressure set during the change of the gear position. Starting from a presettable basic value of the pressure, in the course of the operation of the transmission, the increased pressure is automatically and in steps for each control operation adapted to a target value in such a manner that an input rotational speed of the transmission or a rotational speed of a driving engine connected in front of the transmission follows a preset sequence.
An advantage of the present invention is that it provides a process for controlling the actuating pressure in control elements of an electrohydraulically controlled motor vehicle transmission in which the control elements of the transmission, such as clutches and brakes, in the case of a control, are uniformly filled virtually independently of the operating condition of the motor vehicle. Thus, the power flux between the driving engine and the transmission output is not significantly interrupted. An excessive dropping of the rotational engine speed during the change of the gear position is therefore avoided, particularly in the case of downshifts in the coasting operation.
It is a further advantage that the control element actuates a clutch or a brake of a multi-gear transmission system with planetary trains, and the control element engages at least one gear position of the transmission system, the control element adjusting the pressure in the hydraulic circuit to a variable value which is a function of operating conditions of the motor vehicle and at least of the torque to be transmitted by the transmission system, wherein the increased pressure is a function of the gear positions to be changed and of the direction of the change of gear positions. The rotational speed range of the internal-combustion engine is divided into ranges and a certain pressure level corresponds to the increased pressure for each range.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.